Field
This present subject matter relates generally to an apparatus and method for measuring the wear rate of materials in a high-temperature pressurized environment.
Background
A number of fretting wear machines exist for bench testing material specimens subjected to reciprocating motion. As defined herein, fretting includes high frequency (>20 Hz), low amplitude (10-125 μm) motion, and sliding includes low frequency (1-10 Hz), high amplitude (125-500 μm) motion to test one or more samples pairs in one or more of a fretting and/or sliding motion configuration. These machines are typically configured as pin on disc, ball on flat, block on ring, crossed cylinders, or sliding ball test rigs. Selection will depend on the type of test performed, sample configuration, and/or the specific application. Some of these machines are disclosed in U.S. Pat. No. 3,945,241 (Brown); U.S. Pat. No. 5,375,451 (Sandstrom); U.S. Pat. No. 5,969,226 (Wert); and U.S. Pat. No. 6,601,456 (Davidson).
One drawback of these machines is that they can only test one wear couple at a time, and are generally unsuitable for operation in a pressurized, high temperature environment. They are limited to reciprocating tests in air using simple geometry stationary specimens loaded with dead weights placed on top of the specimen to generate the normal load. Additionally, measuring only one wear couple at a time precludes accurate comparisons between the specimen of interest and its associated control specimen. The most common actuation method used to generate reciprocating motion is a mechanical linkage, where motor rotation is converted into small scale linear displacements. Other methods include eccentric cams and followers, with a rotating shaft in conjunction with a cam action generating oscillatory motion.
With these configurations only discrete displacements are attainable, with stroke length typically adjusted by manually replacing a cam. It is difficult to control displacements at mid-stroke position with existing fretting machines. It is also difficult in machines equipped with cams and followers, which lack a device to control and adjust stroke length while they are running. Further, these devices lack the capability to allow precise adjustment (repeatable positioning to ±2 microns in certain embodiments for aligning wear contact surfaces and/or positioning specimens at a desired mid-stroke (reference) position) or stroke amplitude to compensate for backlash or thermal expansion of the load train while the machine is running.
Although mechanical drive systems having mechanical linkages and cams can provide a simple and cost effective means of stroke actuation, they lack the necessary stiffness for preventing parasitic displacement leading to “false fretting” where the displacement or a significant portion of specimen displacement is reduced by unintended slack in the system. Existing devices have clearances and stiffnesses which, even under moderate loads, mask displacements of 10-125 microns. It is therefore necessary to ensure that machine stiffness is high enough to prevent parasitic displacement leading to “false fretting” where the displacement or a significant portion thereof is masked by unintended machine movement.
Other actuators, such as piezo-electric, electro-magnetic and magnetostrictive actuators, also have significant drawbacks. Piezo-electric actuators, for example, can operate at high frequency but are limited to very small displacement and forces, with displacement limited to approximately 20-80 microns, and force limited to approximately 500-1000 lbf. They can be a cost effective means of actuation for low load applications, with the benefit that the inertia of moving components is very low, therefore minimizing out of balance forces and the need for a high mass fretting wear machine. These benefits come at a design cost, however. In a piezo-electric actuator, for example, force generation is inversely proportional to displacement. Therefore, when force generation is maximum displacement drops to zero. Conversely, at full displacement, no force is generated. Another disadvantage is that the stroke of a piezo-electric actuator is directly proportional to applied voltage. As the stroke of the actuator increases, so does the required voltage, typically from 1-2 kV, making piezo-electric actuators unsuitable for large-scale devices.
Electro-magnetic oscillators are also used as actuators in fretting machines, but they too have their disadvantages. Electro-magnetic oscillators are force generators rather than displacement generators. Because the resisting (frictional) force changes as the test progresses, the loop gain of the system varies, altering the system response. Even though an electro-magnetic actuator is operated with positional feedback, the positional control loop is in cascade with the primary force loop. This makes controlling amplitude and stroke mid position problematic.
Still other fretting machines use magnetostrictive actuators. These actuators rely on new rare earth alloys (e.g. Terfenol-D and/or Galfenol) placed in a magnetic field aligned with the material's magnetostrictive axis. When a DC current is applied, the material proportionally expands, converting electric energy into mechanical motion. They require more power than piezo-electric actuators and cannot generate the same levels of displacement and force as servo-hydraulic actuators.
Thus, while a number of fretting machines designed to perform wear testing in air have been developed for research applications, few can operate in a wide range of fretting parameters, and none are suitable for multi-specimen or autoclave environment testing. A need exists for a fretting machine which overcomes one or more of the limitations described above.